Beamforming is a multi-antenna transmission technology used in antenna array with small intervals and the main principle thereof is to utilize the strong relativity of the array signals and the interfering principle of the waves to produce the radiation pattern with strong directivity, making the main lobe of the radiation pattern self-adaptively point to the DOA (direction of arrival) of the user terminal, such that the signal to noise ratio, the system capacity and the coverage area are improved. Dual layer beamforming technology is applied under the condition of relatively sufficient signal scatterers and is the combination of technologies of smart antenna beamforming and MIMO (Multiple-Input Multiple-Output) spatial multiplexing. In TD-LTE (Time Division-Long Term Evolution) system, the channel symmetry of the upper and bottom rows of TDD (Time Division Duplexing) may be utilized to simultaneously transmit two sets of beamforming data flows, thereby realizing the spatial multiplexing, namely, beamforming.
Current methods for beamforming are mainly EBB (Eigenvalue Based Beamforming) and GOB (Grid Of Beam) methods.
EBB is a method for self-adaptively updating the beam and capable of soundly matching with the channel changes and its process is: the base station utilizes SRS (Sounding Reference Signal) sent by the terminal antenna to even the channel covariance matrix corresponding to a plurality of PRBs (physical resource block) and then decompose the characteristic value to obtain beamforming vectors of two data flows.
However, in the current TD-LTE system, the commercial terminal does not support sending SRS by turns. Therefore, when unable to acquire the signals of 2*I channels (I being the antenna number of the base station) and calculate the beamforming vector by using EBB method to decompose the characteristic value, the base station can only utilize the signals of 1*I channels, thereby resulting in small signal power and bad beam directivity corresponding to the second beamforming vector and making the effect of dual layer beamforming poor.
Directed to the above problems existing in EBB method and considering that currently, the smart antenna adopted at the base station is dual polarized antenna at the same time, polarization diversity may be utilized to obtain the effect of dual layer beamforming and hereby, GOB method is proposed. The process of GOB method is: estimating the DOA (Direction Of Arrival) angle of the terminal firstly, then acquiring a set of GOB beamforming vectors of 1*I (I being the antenna number of the base station) from the pre-configured array vectors according to the DOA angle and finally acquiring two polarized-direction beamforming vectors respectively used for the two data flows.
Compared with EBB method, GOB method has a greater improvement in performance. However, GOB method divides the whole space into L regions, setting an initial angle for each region and the beam shape within each region is fixed. Therefore, GOB method is applicable to LOS (Line-of-sight) channel with a straight ray path. However, for NLOS (Non-Line-of-Sight) channel, the beam acquired by GOB method cannot match with the characteristics of the channel well, thereby resulting in a poor beamforming effect for the NLOS channel.